I'm very happy with this lens. I thought long and hard before buying it. I own a 20D and a 5D and was very hesistant to buy an EF-S only lens and was tempted by the 16-35L.
Anyway, lots of great reviews so I decided to buy it from a store with a good return policy. I had 30 days to put it through its paces and I am very happy with the lens. My copy is very sharp and has excellent color and contrast rendition.
I also felt that the other brands Sigma, Tokina were more of a hit and miss on quality and possibly the Canon would have a better resale value.
I may eventually sell this camera if I replace the 20D with a FF backup but I love my 20D so much, I don't think I will do that. It is nice to have the option of crop vs FF depending on what I am shooting and now if I need wide for the crop, I have it.

light weight. No vignetting of built in flash, good range on crop cameras & quiet focusing. Excellent price.

Cons:

Not as sharp as I like. Dull corners at 20mm. Colors seem a bit dull.

I usually leave my 24-70 f2.8 lens on my camera. But when I want just a light "snapshot" lens, I will throw on this 20-35. It allows me to use the built in flash of my 20D and covers a nice range. For instance taking a group shot at a dinner party is possible at the table using 20mm. The 24mm was just not quite wide enough.

However, when you get used to excellent images from "L" glass, the images from this lens can seem downright bad. They are pretty soft and lack some contrast. However, comparing to L lenses is apples to oranges.

If you strictly view this lens for what it is and the price, it is an excellent lens. It still takes good pictures, focuses well, is lightweight and great price for what you get. Serves as an excellent walk-about lens.

I love this lens. Initially I thought it was too heavy for an everyday lens but this lens stays on my camera about 80% of the time. I thought of trading it for the 24-105f4 because that lens seemed a bit smaller and image quality seemed comparable. But I really do use f2.8-4 quite a bit. I like the blurred backgrounds and ability to shoot in fairly low light so I opted to keep my 24-70.

There are lots of praise for this lens and there is a reason for that. This lens is built like a tank and takes amazing pictures.